Deactivating group

In organic chemistry, an electron donating group (EDG) or electron releasing group (ERG) (+I effect) is an atom or functional group that donates some of its electron density into a conjugated π system via resonance or inductive effects, thus making the π system more nucleophilic. When attached to a benzene molecule, an electron donating group makes it more likely to participate in electrophilic substitution reactions. Benzene itself will normally undergo substitutions by electrophiles, but additional substituents can alter the reaction rate or products by electronically or sterically affecting the interaction of the two reactants. EDGs are often known as activating groups.

An electron withdrawing group (EWG) (-I effect) will have the opposite effect on nucleophilicity as an EDG, as it removes electron density from a π system, making the π system more electrophilic. When attached to a benzene molecule an electron withdrawing group makes electrophilic aromatic substitution reactions slower and more complex, and EWGs are often called deactivating groups. Depending on their relative strengths, EWGs also determine the positions (relative to themselves) on the benzene ring where substitutions must take place; this property is therefore important in processes of organic synthesis.

...
Wikipedia